An image generation system equipped with a scanning electron microscope is widely used for imaging patterns (interconnect trenches, contact holes, etc.) formed on a surface of a wafer. Automatic focusing and automatic astigmatism adjustment in a conventional scanning electron microscope are achieved by acquiring a plurality of images while changing related operation parameters and adopting operation parameters that result in the highest image-quality evaluation value.
One example of the image-quality evaluation is a method of calculating an edge sharpness of a pattern appearing on an image. The edge sharpness is obtained by scanning the image in a direction perpendicular to a direction in which the edge extends to obtain brightness values, calculating derivative values of the brightness values, and adding up the obtained derivative values. In the scanning electron microscope, unlike optical instruments, astigmatism, lens axis alignment, etc. may change. Therefore, it is important to evaluate the image quality by calculating the edge sharpness in multiple directions.
However, since the image generation system does not recognize an actual shape of a pattern, it is impossible to compare the actual shape of the pattern with a pattern appearing on an image. As a result, the image generation system may fail to obtain the optimum values of the operation parameters. Several examples will be described below with reference to drawings.
FIG. 14 is a diagram showing an example of an image of a grid pattern generated by a scanning electron microscope. In this example, edges of the pattern are uniformly present in a vertical direction and a horizontal direction. FIG. 15 is a diagram showing edge sharpness in four directions of 0°, 90°, 45°, and 135° of the pattern image shown in FIG. 14. In this example, the horizontal direction is defined as an angle of 0°. In FIG. 15, the vertical axis represents edge sharpness, and the horizontal axis represents focus parameter. The focus parameter is a parameter for changing the focus of the scanning electron microscope. The grid pattern image entails large edge sharpness in the 0° direction and the 90° direction.
FIG. 16 is a diagram showing another example of an image of the same grid pattern. In this example, there is an astigmatism, and edges in the 0° direction are blurred (specifically, the focal points are different between the 0° direction and the 90° direction). As a result, as shown in FIG. 17, the edge sharpness in the 0° direction is smaller than that in the example of FIG. 15, and only the edge sharpness in the 90° direction has a large peak value. FIG. 18 is a diagram showing an example of an image of a line and space pattern extending in the vertical direction. In this example, as shown in FIG. 19, only the edge sharpness in the 90° direction has a large peak value.
The image shown in FIG. 16 is an image having astigmatism, and it is therefore necessary to adjust an astigmatism correction parameter of the scanning electron microscope, whereas the image shown in FIG. 18 is an image with no astigmatism. However, graphs, each representing a correlation between the edge sharpness and the focus parameter shown in FIGS. 17 and 19, are approximately similar. As a result, the image generation system recognizes the graph shown in FIG. 17 as a normal graph and mistakenly determines that the astigmatism correction parameter is correctly adjusted.